In modern wire bonding machines, the ultrasonic system plays a crucial role in determining bond quality. These machines have a transducer, a bonding tool, and an electrical drive circuit. The ultrasonic system affects the energy transfer to the bond under formation. The interconnection formed between the bonding wire and the work-piece device is a critical aspect of the wire bonding process. In the semiconductor field, the integrity of the interconnection is one of the keys to semiconductor device reliability. Ultrasonic bonding has been used in wire bonding for over forty years and the ultrasonic system has been improved over these years to provide faster, more repeatable, and stronger wire bonds.
The ultrasonic system on a wire bonder machine is an ultrasonic generator and a transducer. The ultrasonic generator provides electrical power to the transducer at a given frequency. On state-of-the-art wire bonders, the ultrasonic generator employs a phase lock loop and amplitude control circuitry to provide automatic adjustment to track the changes during wire bonding. The transducer is a piezoelectric resonator that translates electrical energy from the ultrasonic generator into mechanical vibrations that help form the wire bond connection. The transducer, also called the ultrasonic horn, has a clamping mechanism to which the bonding tool is mounted. In ball bonding, the bonding tool is typically a ceramic capillary. The transducer itself has a driver (piezoelectric crystals), an amplifier (tapered body for amplitude amplification), and mounting flanges for mounting the transducer to the bondhead of the wire bonding machine.
Bonding is accomplished by applying an electrical load to the piezoelectric crystals causing ultrasonic vibrations that cause the tapered body of the horn to vibrate in the lengthwise direction (axial direction). At the same time, the ceramic capillary is lowered to contact the work-piece device.
It is known to mount the transducer to the wire bonding machine so that the transducer would be precisely located at a theoretical node point or zero displacement point in relation to its vibrational frequency. At this position, oscillation during the raising and lowering (axial movement) of the tapered body of the ultrasonic horn can be effectively prevented. This zero node mounting would couple the least amount of energy from the piezoelectric crystals into the bonding machine to which the transducer was mounted. Some transducers are manufactured as a unibody design, that is, the mounting flanges and tapered body are one piece. These transducers are limited in that they have only been able to operate at single nodes or harmonics thereof, called fixed-frequency transducers. When such prior art fixed-frequency transducers are driven at multiple frequencies (non-harmonic), several problems arise which have prevented such prior art transducers from being useful at the additional frequencies.
Moreover, even while these fixed-frequency transducers operate at their specific nodes, there is still some portion of their mounting ears that are not positioned at the frequency nodal points. As a result, the transducer mounting ears vibrate and cause energy dissipation. In order to reduce this energy loss in a unibody transducer, the mounting flanges are made extremely thin to stay at the nodal points. If the mounting flanges are too thin, however, there is a corresponding drop in the stiffness of the vibrating horn due to the weak mounting flanges. To solve this problem, a transducer with mounting flanges separate from the transducer body have been employed. Unfortunately, by introducing separate structures to connect the mounting flange to the transducer body, the stiffness of the transducer body is compromised.
What is needed therefore is an ultrasonic transducer suitable for use at multiple ultrasonic frequencies having mounting flanges of sufficient stiffness that do not impede ultrasonic vibrations in the transducer and at the same time prevent vibrations from being transmitted into the machine bond head.